The present invention relates to a process and an apparatus for the deposition of anti-reflection coatings and the checking of their thickness. It is more particularly used in the production of non-resonating semiconductor laser amplifiers for use in optical telecommunications.
It is known that an anti-reflection coating is a transparent coating, whose index end satisfies the relation n.sup.2 =n.sub.0, in which n.sub.0 is the index of the substrate on which it is deposited and whose thickness e is equal to .lambda./ 4n, in which .lambda. is the wavelength of the radiation for which it is wished to eliminate the reflection.
For a given material, the coating thickness must be carefully adjusted. It is accepted that a "good" anti-reflection coating is one having a reflectivity below 10.sup.-3. This very low value imposes severe constraints on the thickness, which must be adjusted to better than 10.sup.-2. It is therefore relatively difficult to obtain good anti-reflection coatings.
Processes for checking the thickness of such coatings when they are deposited on the lateral faces of semiconductor structures are known. The article entitled "Real-time monitoring of laser diode facet reflectivity while being coated with SiO.sub.x ", published by A. SOMANI et al, in "Applied Optics", vol. 27, No. 8, April 1988, pp 1391-1393, describes a process in which a measurement is made of the light intensity emitted by the laser diode as a function of time. When this intensity passes through a minimum, the reflectivity also passes through a minimum and the deposition process is interrupted. U.S. Pat. No. 3,846,165 describes a process in which measurement takes place of the intensity of the radiation emitted by the laser, which is a function of the reflectivity of the deposited coating. When the measured power corresponds to the desired reflectivity, the deposition process is interrupted. The article by M. SERENYI et al entitled "Directly controlled deposition of anti-reflection coatings for semiconductor laser" published in "Applied Optics", Mar. 1, 1987, vol. 26, No. 5, pp 845-849, again describes a process in which the semiconductor structure is supplied with current and the spontaneous light intensity emitted through the coating during deposition is measured. A maximum lighting power is then sought.
All the above procedures suffer from numerous disadvantages. Firstly, the photoreceiver required must be positioned in such a way as to collect a maximum of light. However, this requirement is with the need of not masking the face to be treated. As this photoreceiver is sensitive to the light emitted in the deposition enclosure (by the sputtering gun or plasma), it is necessary to mask this parasitic light and use interferometric filters for this purpose, which reduce the lighting intensity to be measured. This reduction is all the more prejudicial, because at its minimum the intensity to be measured is already very low. Finally, there must be a strict alignment between the laser and the photoreceiver using lenses, mirrors, optical fibers, etc., all these means significantly complicating the installation.
Moreover, when these techniques are applied to non-resonating structures, which require that the two faces are given an anti-reflection treatment, new difficulties are encountered. Thus, on optimizing the thickness of the deposited coating one face after the other, a thickness asymmetry is created between the two finally-deposited coatings. This is due to the fact that during the anti-reflection treatment of the first face, the increase in the optical losses induces a spectral slip of the optical gain of the structure, which leads to a slightly different optimum thickness for the second face. In addition, during the treatment of the second face, a parasitic deposition is added to the already-treated first face, which will upset the first treatment.